Releasable locking mechanism for axial piston hydraulic pump

ABSTRACT

A positive displacement hydraulic pump/motor assembly ( 1 ) includes a rotary cylinder block ( 3 ) having a central axis ( 5 ) and a generally circular array of cylinders ( 6 ) disposed in parallel relationship around the axis. A corresponding plurality of axial pistons ( 10 ) is reciprocably disposed within the respective cylinders. A drive shaft ( 12 ) effects rotation of the cylinder block about the central axis and a drive plate ( 15 ) is disposed at one end of the cylinder block ( 3 ) to effect sequentially staggered reciprocation of the pistons ( 10 ) in response to rotation of the cylinder block. A stationary valve plate ( 20 ) is disposed at an opposite end of the cylinder block. The valve plate ( 20 ) includes a valve face ( 21 ) adapted for sliding rotational engagement with a complementary mating face ( 22 ) formed on the cylinder block. The valve plate further includes at least one inlet port ( 24 ) adapted for fluid communication with a source of hydraulic fluid and at least one outlet port ( 26 ) adapted for fluid communication with an hydraulic load. The ports ( 24, 26 ) are disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate. The pump/motor assembly further includes selectively releasable coupling means ( 50 ) adapted in an engaged mode to connect the drive shaft to the cylinder block and in a disengaged mode to decouple the drive shaft from the cylinder block.  
     The pump/motor assembly is adapted for incorporation into an energy management system operable in a driving mode, a braking mode and a neutral motor to provide supplementary drive from regenerative braking in a vehicle.

FIELD OF THE INVENTION

[0001] The present invention relates generally to hydraulic motors and pumps, and more particularly to positive displacement axial piston motors and pumps.

[0002] The invention has been developed primarily for use with a pump/motor assembly which forms part of a regenerative drive system (“RDS”), and will be described predominantly hereinafter in that context. It should be appreciated, however, that the invention is not limited to this particular field of use, being readily adaptable to any axial piston hydraulic motor or pump for use in virtually any application.

BACKGROUND OF THE INVENTION

[0003] In the present context, the invention has been developed more specifically as an improvement to the RDS described by the present applicant in an earlier patent application filed via the Patent Cooperation Treaty (PCT) as international application No PCT/AU99/00740, the full contents of which are hereby incorporated by reference.

[0004] As previously described in that earlier patent application, the RDS is based upon a positive displacement pump/motor arrangement incorporating a cylinder block which houses a cylindrical array of axially reciprocating pistons. In one preferred embodiment, the cylinder block and valve face are coaxially disposed around the primary drive shaft, thereby avoiding the need for intermediate gearing, chains, belts or other transmission elements. When used in conjunction with a suitable accumulator, the resultant regenerative drive system provides a practical and commercially viable system for harnessing the previously wasted braking energy of a vehicle, storing this energy, and subsequently releasing it back into the drive train as required under conditions of acceleration, load, or gear change transitions.

[0005] This RDS arrangement significantly improves the overall efficiency of the engine and power transmission systems of the vehicle. The RDS system also conveniently acts as an efficient auxiliary braking mechanism in the energy accumulation mode. The system as described, however, is subject to several significant limitations, many of which are common to previously known axial piston hydraulic motors and pumps.

[0006] In this regard, one significant limitation in the pump/motor assembly as previously described, relates to the inherent drag associated with the seals, bearings, valve faces and other elements that are in direct sliding contact with each other, as the cylinder block and other components in the rotational group rotate with respect to the valve plate, housing and other stationary components of the system. In the particular RDS unit described, these frictional drag forces are reduced by virtue of the fact that most of the components in direct contact with one another “float” on a film of oil, which is often pressurised. Nevertheless, a residual drag factor remains, which consumes power, generates heat, and compromises the potential efficiency of the system.

[0007] From this perspective, the present invention is concerned more specifically with the drag normally attributable to the direct communication between the rotational group including the cylinder block, and the valve plate. In the pump/motor assembly and RDS unit as previously described in the earlier patent application referenced above, a “hold-down” spring is disposed resiliently to bias the cylinder block axially into face-to-face sliding engagement with the valve plate. Because of the relatively high pressures generated within the pump/motor assembly, the hold-down spring is required to exert a relatively high degree of axial force on the cylinder block in order to prevent excessive fluid leakage between the block and the valve plate.

[0008] While effective in preventing excessive leakage, the relatively high axial force between the cylinder block and the valve plate causes significant frictional drag between these components as they slide rotationally relative to one another, separated only by a thin film of pressurised oil. The associated inefficiency is particularly significant in situations where the pump/motor assembly is operating in a neutral or “free-wheel” mode, in which the unit is neither pumping nor driving but is nevertheless rotating, often at high speed, as a result of direct connection with a rotary source of power or load.

[0009] While applying to some extent in almost any application of axial piston hydraulic motors and pumps, this limitation is particularly significant in the context of an RDS unit fitted to a vehicle, in the manner previously described. That is because in many situations, the RDS may “free-wheel” for cumulative periods of many hours. During this time, the unit may generate neither motor nor pump pressure, but would nevertheless continue to rotate at the same speed as the vehicle drive line to which it is integrally connected. This would predominantly occur, for example, during long runs on open highways with minimal changes in traffic conditions or road topography. In such situations, the RDS may do no effective work for prolonged periods, and yet introduce an inherent drag factor into the drive line, which ultimately compromises the efficiency of the power train of the vehicle.

[0010] Moreover, the basic problem is compounded in such conditions because the drag factor itself is exacerbated when the pump/motor unit is operating in the neutral or free-wheeling mode. This is because the unit is not producing hydraulic pumping pressure sufficient to sustain the film of hydrostatic pressurised oil required to minimise the effect of frictional drag between the components in direct rotational contact with one another.

[0011] The foregoing discussion of the prior art is intended solely to place the invention in an appropriate context, and allow a proper appreciation of its technical significance. Any statements made in this specification about prior art information should not be construed as admissions that such information is widely known, or forms part of common general knowledge in the relevant field.

[0012] It is an object of the present invention to overcome or substantially ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.

DISCLOSURE OF THE INVENTION

[0013] Accordingly, the invention provides a positive displacement hydraulic pump/motor assembly including:

[0014] a rotary cylinder block having a central axis and incorporating a generally circular array of cylinders disposed in parallel relationship around the axis;

[0015] a corresponding plurality of axial pistons reciprocably disposed within the respective cylinders;

[0016] drive means to effect rotation of the cylinder block about the central axis;

[0017] a drive plate disposed at one end of the cylinder block to effect sequentially staggered reciprocation of the pistons in response to rotation of the cylinder block;

[0018] a stationary valve plate disposed at an opposite end of the cylinder block, the valve plate having a valve face adapted for sliding rotational engagement with a complementary mating face formed on the cylinder block;

[0019] the valve plate further including at least one inlet port adapted for fluid communication with a source of hydraulic fluid and at least one outlet port adapted for fluid communication with an hydraulic load;

[0020] the ports being disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate;

[0021] the pump/motor assembly further including selectively releasable coupling means adapted in an engaged mode to connect the drive means to the cylinder block and in a disengaged mode to decouple the drive means from the cylinder block.

[0022] It will be appreciated that the same assembly may be used in one mode as a motor, and in another mode as a pump. It should therefore be understood that throughout the specification, these terms may be used in conjunction, or interchangeably. In each case, however, unless the context clearly dictates otherwise, any reference to configuration of the invention as a pump should be understood to include configurations as a motor, and vice versa. It should also be understood that the inlet and outlet ports may alternate in function according to the mode of operation of the unit.

[0023] It should further be understood that the terms “sealing”, “sealing engagement” and the like are intended to convey the sense of prevention of excessive leakage past or through the relevant components. It will be appreciated, however, that in a system of this nature, a minimal level of leakage flow may persist, and indeed maybe desirable for lubrication purposes, notwithstanding the fact that effective sealing, in the intended sense, has been achieved.

[0024] Preferably, the drive means include a drive shaft, disposed in coaxial relationship with the cylinder block. Most preferably, the drive shaft extends through a complementary bore formed in the cylinder block. The assembly preferably further includes coupling means disposed drivingly to connect the shaft to the cylinder block. The coupling means may be fixed or selectively releasable. In one embodiment, the shaft is splined or keyed to the cylinder block within the bore.

[0025] In a particularly preferred embodiment, the positive displacement pump/motor is a swash plate type unit. In this embodiment, the drive plate takes the form of a stationary swash plate, which is inclined with respect to the central rotational axis of the cylinder block. Preferably also, the ends of the pistons remote from the valve plate include “followers” adapted to slide over the swash plate as the cylinder block rotates. A hold-down plate is preferably disposed to capture the floating ends of the pistons and retain the followers in sliding contact with the swash plate. In alternative embodiment, however, springs or other suitable means may be used to retain the followers in contact with the swash plate.

[0026] Preferably, the angle of inclination of the swash plate is selectively adjustable, to provide variable flow rate characteristics. In particular, the swash plate is preferably adapted to be selectively inclined in a positive or a negative sense, thereby enabling the assembly alternately to operate as a motor or a pump. Most preferably, the variable swash plate can also be oriented in an intermediate or neutral position, effectively normal to the central axis, such that rotation of the cylinder block causes no movement of the pistons, hence induces no net flow into or out of the cylinders through the ports, and therefore causes no load on the system aside from a residual level of inherent frictional drag.

[0027] In other embodiments, it will be appreciated that the invention may also be applied to a bent axis type hydraulic pump. In that case, connecting rods for the pistons are pivotably attached to a thrust plate adapted to rotate with the cylinder block. The invention may also be adaptable to other configurations of motors and pumps.

[0028] Preferably, the pump/motor assembly further includes selectively variable bias means disposed to apply a variable bias force urging the respective mating faces on the cylinder block and the valve plate into sealing engagement.

[0029] Preferably, the bias means include an hydraulic hold-down piston disposed within a complementary hold down cylinder. An electronically activated hydraulic valve actuator is preferably also provided to regulate pressure in the hold-down cylinder.

[0030] The hold-down cylinder is preferably supplied by pilot pressure from the pump/motor assembly. Alternatively, however, it may be supplied from an external source of hydraulic fluid pressure. It will also be appreciated that the hold-down piston need not be hydraulic, but could alternatively be operated by means of an electromagnetic solenoid, a mechanical screw, or other suitable electrical, magnetic, electromagnetic, mechanical, hydraulic, pneumatic or hybrid arrangements.

[0031] In the preferred embodiment, the hydraulic hold-down piston takes the form of an annular sleeve disposed coaxially around the drive shaft, at the end of the cylinder block remote from the valve plate.

[0032] The valve actuator is preferably adapted to supply pressurised oil into the hydraulic hold-down cylinder, and release oil from that cylinder, according to the pressure and flow characteristics of the pump/motor assembly. In general, the system is ideally regulated such that under conditions of minimal motor or pump load, the pressure to the hold-down cylinder is released or substantially reduced so as to minimise frictional drag between the rotary cylinder block and the stationary valve plate. Conversely, under conditions of relatively high motor or pump load, the supply pressure to the hold-down cylinder is substantially increased, so as to minimise leakage from the ports at the interface between the cylinder block and the valve plate.

[0033] This pressure regulation may be controlled in a linear, step-wise or other manner in response to pressure changes at selected points in the system. It may also be controlled wholly or in part according to other system parameters such as leakage flow, frictional drag, temperature changes or the like, as well as various combinations of such parameters.

[0034] Preferably, in addition to the variable hold-down mechanism, the assembly includes a non-adjustable retention spring disposed to provide a minimum level of substantially constant initial bias force sufficient to hold the cylinder block against the valve plate at a relatively low threshold pressure. In the preferred embodiment, the retention spring takes the form of a preloaded annular bevel or frusto-conical washer disposed within an annular recess formed between the hold-down piston and the cylinder block.

[0035] Preferably, the threshold force provided by the retention spring is sufficient to allow the pump to supply a predetermined level of pilot pressure to the hydraulic hold-down cylinder with minimal leakage, at an initial positive or negative (but non-zero) swash plate angle. This initial swash plate angle for the desired level of pilot pressure is preferably between 0.1 degrees and around 5.0 degrees, more preferably between 0.3 and around 1.0 degrees, and ideally around 0.5 degrees off the neutral or zero position.

[0036] In a supplementary aspect, the invention preferably further includes a selectively operable hydraulic lift-off cylinder effectively interposed between the cylinder block and the valve plate, such that upon actuation, the cylinder block is axially displaced marginally away from the valve plate. Preferably, the exposed end of the lift-off cylinder is in direct contact with a thrust bearing, to facilitate continued rotation of the cylinder block with minimal frictional resistance. The lift-off cylinder would normally only be activated with the swash plate in the neutral position, when there is minimal internal pressure within the pump/motor assembly. Advantageously, this lift-off mechanism enables selective minimisation of frictional drag between the cylinder block and the valve plate in situations where sealing of the valve ports in order to prevent leakage flow is non-critical to performance, because the pump/motor unit is doing no work.

[0037] In the preferred embodiment, the coupling means include a locking mechanism, desirably in the form of a tapered locking collet disposed coaxially around the drive shaft. The locking collet is desirably splined internally for engagement with a correspondingly splined section of the shaft, so as to transmit rotary drive, while accommodating a limited degree of relative axial displacement between the collet and the shaft.

[0038] Preferably, the hydraulic hold-down cylinder, in addition to its function of applying a variable hold-down force, is configured to positively disengage the locking collet mechanism when the hydraulic lift-off cylinder is simultaneously de-pressurised.

[0039] The inner surface of the cylinder block is preferably tapered to match the outer surface profile of the locking collet. The outer surface of the locking collet is preferably also splined, for engagement with complementary splines formed on the inner surface of the cylinder block such that upon axial engagement, the two components become mechanically interlocked for conjoined rotation. In this way, the locking collet positively transmits rotary drive from the shaft to the cylinder block, while still allowing a limited degree of axial displacement of the rotational group along the shaft. This limited degree of axial displacement, with the splines engaged, preferably permits the cylinder block to be alternately held positively against the valve plate by the hold-down cylinder and displaced marginally away from the valve plate by the lift-off cylinder. Upon full axial disengagement, however, the shaft is able to spin independently of the surrounding rotational group. Advantageously, this mechanism selectively allows frictional and hydrodynamic drag to be further reduced in situations where the pump/motor assembly is not under load.

[0040] In a variation of this embodiment, other forms of interlocking engagement may be used as an alternative to splines, to selectively transmit drive between the shaft and the cylinder block. It will also be appreciated that the outer surface of the locking collet may not be splined or otherwise mechanically keyed for interlocking engagement with the mating inner surface of the surrounding cylinder block. Rather, the locking mechanism may rely upon progressive, albeit less positive, frictional engagement.

[0041] In one particularly preferred application, the invention is adapted for incorporation into an energy management system operable in a driving mode, a braking mode and a neutral mode, the energy management system including:

[0042] energy accumulation means operable selectively to store and release energy through controlled receipt and release of hydraulic fluid;

[0043] a positive displacement hydraulic pump/motor assembly as defined above, in fluid communication with the energy accumulation means;

[0044] an hydraulic reservoir in fluid communication with the pump/motor assembly; and a drive shaft;

[0045] the system being arranged such that in the braking mode the pump/motor assembly retards the drive shaft by pumping hydraulic fluid into the accumulation means, in the driving mode the pump/motor assembly supplies supplementary power to the drive shaft using pressurised hydraulic fluid from the accumulation means, and in the neutral mode the pump/motor assembly is effectively inoperative and exerts no substantial driving or retarding influence on the drive shaft.

[0046] In one preferred implementation of this aspect of the invention, the drive shaft forms part of the drive train of a vehicle. Most preferably, the drive shaft extends through a complementary bore formed in the cylinder block, such that the drive shaft and the cylinder block are coaxial, the pistons of the pump/motor assembly are uniformly disposed in parallel relationship around the drive shaft. In this embodiment, the coupling means preferably include a splined connection directly

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0048]FIG. 1 is a cross-sectional view of a pump/motor assembly, showing a hold-down cylinder activated in the normal operating or loaded configuration, according to a first embodiment of the invention;

[0049]FIG. 2 is a cross-sectional view similar to FIG. 1, showing the hold-down cylinder in the unloaded or deactivated configuration;

[0050]FIG. 3 is a cross-sectional view showing a pump/motor assembly incorporating a lift-off cylinder shown in the normal operating (deactivated) mode, according to a second embodiment of the invention (NB—For clarity, this version of the pump/motor unit as illustrated substitutes a conventional spring for the hold-down cylinder shown in FIGS. 1 and 2);

[0051]FIG. 4 is a cross-sectional view of the pump/motor unit of FIG. 3, showing the lift-off cylinder in the unloaded (activated) mode;

[0052]FIG. 5 is a cross-sectional view showing a pump/motor assembly incorporating a hold-down cylinder, a lift-off cylinder and a tapered locking collet mechanism in the normal operating (locked) mode according to a third embodiment of the invention;

[0053]FIG. 6 is a cross-sectional view of the pump/motor unit of FIG. 5, showing the locking collet mechanism in the unloaded (unlocked) mode;

[0054]FIG. 7 is a diagrammatic perspective view showing a pump/motor assembly of the general type illustrated in FIGS. 1 to 6, incorporated into a road vehicle as part of a regenerative drive system (RDS), according to a further aspect of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

[0055] Referring initially to FIGS. 1 and 2, the invention provides a positive displacement hydraulic pump/motor unit 1. The pump/motor unit includes a stationary housing 2 and a cylinder block 3 supported within the housing for rotation about a central axis 5. The block 3 incorporates a circular array of hydraulic cylinders 6 uniformly disposed in parallel relationship about the central rotational axis 5. A corresponding array of axial pistons 10 is reciprocably disposed within the respective cylinders.

[0056] A central drive shaft 12 extends through a complementary bore 13 formed in the cylinder block. The shaft is drivingly connected to the cylinder block 3 by coupling means including spline formations 14 to effect rotation of the block about the central axis, as described in more detail below.

[0057] A stationary drive plate in the form of swash plate 15 is disposed at one end of the cylinder block (the right-hand end when viewing the drawing). The swash plate is pivotably supported within the housing, for adjustable movement within a predetermined range, about an axis substantially normal to the rotational axis of the cylinder block.

[0058] A hold-down plate 16 is disposed to locate the free ends of the pistons remote from the valve plate in the appropriate relative spatial relationship, while the end faces of the pistons are formed with followers 18 adapted to engage and slidably traverse the operative surface of the swash plate. In this way, rotation of the cylinder block effects sequentially staggered reciprocation of the pistons, with the amplitude of piston travel being determined by the selected angle of inclination of the swash plate.

[0059] A stationary valve plate 20 is disposed at the opposite end of the cylinder block (the left-hand end when viewing the drawings) and is rigidly connected to the housing. The valve plate includes a valve face 21 adapted for sliding rotational engagement with a complementary mating valve face 22 formed on the abutting end of the cylinder block. The valve plate includes inlet ports 24 adapted for communication with a source of hydraulic fluid, and outlet ports 26 adapted for fluid communication with an hydraulic load.

[0060] The valving is arranged such that hydraulic fluid is progressively drawn into the cylinders in sequence through the inlet ports as the pistons withdraw away from the valve plate and is subsequently expelled from the cylinders through the outlet ports as the respective pistons are progressively advanced toward the valve plate, under the influence of the swash plate.

[0061] The swash plate is pivotably supported within the housing such that the effective angle of inclination with respect to the rotational axis of the cylinder block is adjustable to provide selectively variable flow characteristics. In particular, the swash plate may be alternately inclined in a positive and a negative sense, thereby enabling the assembly selectively to operate as a motor or a pump. In this regard, it should be appreciated that the particular valve ports which function as inlets to the cylinders of the pump/motor unit, and those which function as outlets, will alternate according to the operational mode of the unit. Importantly, the swash plate can also be orientated in an intermediate or neutral position in a plane effectively normal to the central axis, such that rotation of the cylinder block produces no reciprocation of the pistons. In this mode, the pump/motor unit induces no net fluid flow into or out of the cylinders, and consequently transfers no significant hydraulic load to the shaft.

[0062] The essential elements of construction and the basic principles of operation are common to most positive displacement axial piston hydraulic pumps, and being well understood by those skilled in the art, need not be described in more detail.

[0063] The version of the pump/motor unit as illustrated further includes controllable bias means disposed to apply a variable bias force urging the mating faces 21 and 22 on the valve plate and the cylinder block respectively into sealing engagement. The bias means include an hydraulic hold-down piston 30 disposed within a complementary hold-down cylinder 32. As best seen in FIGS. 1 and 2, the hold-down piston 30 takes the form of an annular sleeve disposed coaxially around the drive shaft, at the front (right-hand when viewing the drawings) end of the cylinder block, remote from the valve plate. The hold-down cylinder is correspondingly shaped, and is pressurised via a supply passage 35, which extends through the housing and the drive shaft as shown.

[0064] The pressure supplied to the hold-down cylinder is regulated by an electronic valve actuator 36 (represented diagrammatically in FIG. 1 only) according to predetermined operational parameters related to the pressure and flow characteristics of the pump/motor assembly and other characteristics of the system, which will vary according to the application.

[0065] In general terms, the hold-down cylinder is regulated such that under conditions of relatively high motor or pump load, the supply pressure to the hold-down cylinder is maintained at a predetermined level, sufficient to minimise leakage from the ports between the mating valve faces at relatively high pump/motor pressures. This is the normal operating mode for the unit, as shown in FIG. 1. Conversely, under conditions of minimal motor or pump load, the pressure to the hold-down cylinder is released or substantially reduced, so as to minimise frictional drag at the sliding interface between the rotary cylinder block and the stationary valve plate. This is the unloaded or passively released mode of operation, as illustrated in FIG. 2. In this mode, hydraulic fluid is exhausted from the hold-down cylinder via passage 35, as indicated by the directional arrows.

[0066] Within these limits, the pressure may be regulated by the valve actuator 36 in a linear, stepwise or other manner in response to pressure changes at selected points in the system. It may also be regulated wholly or in part according to other parameters such as measured leakage flow, frictional drag, temperature changes or the like, as well as various combinations of such parameters on the basis of a suitable control system. This enables an optimum balance to be achieved, on a dynamic basis, between the need to minimise valve leakage on one hand, and the desirability of minimising internal friction and hydrodynamic drag on the other. In previously known pumps of this type, the cylinder block was simply biased toward the valve plate by a spring applying a substantially constant preload force, which if calibrated for maximum pressure conditions would cause significant frictional drag and hence inefficiency at other times, or if calibrated for intermediate pressure levels, would allow excessive leakage under high-pressure conditions.

[0067] In addition to the hydraulic hold-down piston and cylinder arrangement 30 and 32, the pump/motor unit ideally includes a non-adjustable retention spring mechanism, with a relatively low level of substantially constant preload, calibrated to provide a minimal threshold level of initial bias force. In the preferred embodiment as illustrated, the retention mechanism takes the form of a preloaded annular bevel or frusto-conical spring washer 39 disposed within a complementary annular recess 40 formed between the hold-down piston and the cylinder block. The bevel spring washer 39 is shown in a flattened configuration in FIG. 1, corresponding to the normal operational or loaded mode, and in a marginally expanded configuration in FIG. 2, corresponding to the unloaded or passively released operational mode.

[0068] The initial bias force of the spring washer 39 is calibrated to hold the cylinder block against the valve plate with a relatively low level of force in the unloaded or released mode of the hold-down cylinder, sufficient simply to allow the pump to begin supplying pilot pressure to the hold-down cylinder via passage 35 at an initial positive or negative (but non-zero) swash plate angle. This initial swash plate angle corresponding to the desired level of pilot pressure is preferably between 0.1 and around 5.0 degrees, more preferably between 0.3 and around 1.0 degrees, and ideally around 0.5 degrees off the neutral or zero position.

[0069] Referring now to FIGS. 3 and 4, wherein similar features are denoted by corresponding reference numerals, a second embodiment of the invention additionally includes a selectively operable active release or lift-off mechanism in the form of an hydraulic lift-off piston 40, slidably disposed within an associated lift-off cylinder 41. The lift-off piston and cylinder are again annular in configuration, and positioned coaxially around the central drive shaft. This hydraulic lift-off mechanism is intended primarily to complement the operation of the hold-down cylinder 32 and the associated retention spring 39 mechanism as illustrated in the first embodiment of the invention shown in FIGS. 1 and 2. For clarity of illustration, however, the lift-off mechanism is shown in FIGS. 3 and 4 in conjunction with a conventional spring-based hold-down mechanism 42 (to which the lift-off mechanism is equally applicable).

[0070] The lift-off piston and cylinder are effectively interposed between the cylinder block and the valve plate. In the normal operating mode, as shown in FIG. 3, the lift-off cylinder is depressurised through supply passage 44, and has no effect on the operation of the pump/motor unit. Upon actuation, however, the lift-off cylinder is pressurised again through supply passage 44, whereby the cylinder block is axially displaced away from the valve plate. This is the positively unloaded or actively released mode of operation, as represented in FIG. 4.

[0071] It will be appreciated that the pressure generated within the lift-off cylinder, in order to effect positive release of the cylinder block, must be sufficient at least substantially to overcome the threshold bias force of the retention spring 38 or 42 associated with the relevant hold-down mechanism. It will also be appreciated that in the situation where the lift-off cylinder is activated without the inclusion of a hold-down cylinder which may be simultaneously depressurised, the lift-off cylinder must activate against a thrust bearing to allow for the continued rotation of the cylinder block with the shaft.

[0072] In practice, the extent of axial displacement may be marginal, and indeed may even be visually indiscernible. The significant result, however, is that the axial lift-off mechanism enables selective minimisation of frictional drag between the cylinder block and the valve plate in situations where the pump/motor unit is doing no work. Of course, the lift-off cylinder would normally only be activated with the swash plate in the neutral position, when there is consequently minimal internal pressure within the pump/motor assembly. Otherwise, the axial displacement of the cylinder block away from the valve plate, albeit by only a small amount, may result in excessive leakage of pressurised hydraulic fluid past the valve ports.

[0073] A further embodiment of the invention is illustrated in FIGS. 5 and 6 where again similar features are denoted by corresponding reference numerals. In this version, it will be seen that cylinders 32 and 41 are incorporated into the pump/motor unit. However, these cylinders operate somewhat differently than those previously described and the coupling means additionally include a releasable locking mechanism 50, to enable selective disengagement of the cylinder block and the associated components in the rotational group, from the central drive shaft.

[0074] Although an essential aspect of the present invention, this selectively releasable locking mechanism is omitted from the earlier embodiments as illustrated, to facilitate the initial explanation of the surrounding components in the assembly and the supplementary aspects of the invention. In the most preferred implementation of the invention as shown in FIGS. 5 and 6, the supplementary and primary aspects are integrated together because the resultant mechanical interaction gives rise to significant synergies. At the same time, however, it should be appreciated that the locking mechanism may be implemented in various forms independently, as well as in conjunction with one or both of, the ancillary hold-down and lift-off mechanisms.

[0075] More particularly, the locking mechanism 50 as illustrated includes a tapered locking collet 52, which is splined internally for engagement with the corresponding spline formations 14 on the drive shaft. The splined connection transmits rotary drive while accommodating a limited degree of relative axial displacement between the locking collet 52 and the shaft. The inner surface 54 of the cylinder block is shaped to match the tapered external surface profile of the locking collet and these tapered surfaces also incorporate complementary axially interengageable spline formations 55. Additionally, and annular sleeve 60 incorporating a shoulder flange 62 is interposed between the hold-down piston 30 and the locking collet 52

[0076] In this embodiment, in the normal operating (locked) mode as shown in FIG. 5, the cylinder 41 is pressurised via passage 44, which urges that piston to the right (when viewing the drawings) while simultaneously urging the cylinder block to the left, toward the valve plate. As the piston 40 advances it drives the locking collet 52, the annular sleeve 60 and the piston 30 axially and in unison to the right, away from the valve plate. In order to accommodate this movement, cylinder 32 is simultaneously depressurised via passage 35. However, when the annular end face 64 of the piston 30 abuts the adjacent end of the cylinder 32, no further axial displacement is possible. From this point, further pressurisation of cylinder 41 through passage 44 progressively drives the cylinder block into sealing engagement with the valve plate in order to perform the hold-down function. At the same time, it will be appreciated that the piston 40 and cylinder 41 operate to retain the splines 55 on the locking collet and the surrounding cylinder block in interlocking engagement. In conjunction with the splined connection between the locking collet and the shaft, this effectively locks the cylinder block, the locking collet and the shaft together for conjoined rotation.

[0077] In order to unload the cylinder block from the valve plate and at the same time to positively unlock the cylinder block from the shaft, the cylinder 41 is vented via passage 44 while the cylinder 32 is simultaneously pressurised via passage 35, as shown in FIG. 6. In this configuration, the end face and 66 of the annular sleeve 60 engages the adjacent face of the locking collet, and drives the locking collet axially toward the valve plate 20 (to the left when reviewing the drawings).

[0078] During an initial phase of displacement, the locking collet carries the cylinder block with it. However, once the cylinder block firmly engages the valve plate, no further axial displacement is possible. Upon further pressurisation of cylinder 32, the piston 30, sleeve 60, and collet 52 are displaced progressively toward the valve plate while the cylinder block is held back, until the piston 40 reaches the end of its travel, at which point no further axial displacement of the locking collet is possible. At this point, the previously intermeshing tapered splines 55 on the locking collet and the surrounding cylinder block are disengaged, to permit relative rotation between the rotational group and the drive shaft. In this position, the cylinder block is both unloaded and unlocked, as represented by the configuration shown in FIG. 6. In this version, it will also be noted that coil springs 68 are incorporated within the cylinders to bias the respective pistons into contact with the swash plate, thereby minimising free play and noise within the pump/motor unit, particularly when operating in the unloaded or unlocked mode.

[0079] It will be appreciated that this mechanism, when unlocked in situations where the pump/motor assembly is not under load, effectively eliminates frictional drag between the cylinder block and the valve plate. It also eliminates drag between the cylinder block and other stationary elements within the housing. This feature is particularly advantageous in applications where the cylinder block rotates within an oil bath and consequently, the hydrodynamic drag on the rotational group is relatively high. In some larger scale applications, however, particularly where there is mechanical interlocking engagement between the drive shaft and the rotational group, it may be desirable to provide an additional control mechanism adapted to spin the cylinder block up to the approximate speed of the drive shaft, so as to minimise transient shock loads at the point of initial engagement of the locking mechanism.

[0080] The present invention in its various forms as illustrated is particularly well adapted for implementation as part of a regenerative drive system (RDS) of the type previously described by the present applicant in the earlier application No. PCT/AU99/00740. An example of one such implementation in a vehicle chassis 70 is illustrated in FIG. 7.

[0081] Referring to FIG. 7, the hydraulic pump/motor assembly 1, as described above, forms part of an energy management system 72, wherein the drive shaft 12 of the pump/motor unit is connected to the drive line or power train 74 of the vehicle via yokes 76 and 78. In situ, these yokes form parts of universal joints at the respective ends of the drive shaft. In this way, the drive shaft of the pump/motor unit becomes serially connected with, and an integral part of, the drive line of the vehicle, thereby obviating the need for gearboxes, chain drives, belt drives, or other intermediate transmission mechanisms. This makes the unit efficient, compact, reliable, and readily retrofitable to existing vehicles.

[0082] In this implementation, the system further includes energy accumulation means in the form of a pair of gas/liquid accumulators 80, each comprising a double-ended cylinder 82 and a piston (not shown) adapted sealingly to float within the cylinder. One side of each cylinder contains a compressible inert gas such as nitrogen, while the other side of the cylinder is in fluid communication with the pump/motor unit via hydraulic lines. Each accumulator is thereby adapted to store energy by receiving pressurised hydraulic fluid into one side of the cylinder so as to compress the gas on the other side, and adapted subsequently to release that energy by expulsion of the hydraulic fluid as the compressed gas is allowed to expand. This method of energy accumulation and regeneration is well understood by those skilled in the art, and is described in more detail in PCT/AU99/00740. Again, however, it should be emphasised that alternative forms of energy accumulator such as bladder or diaphragm type accumulators can readily be substituted and further, that any suitable number of accumulators may be used.

[0083] In use, the system is selectively operable in any one of three primary modes. In a first braking mode, the system operates to retard the drive shaft of the vehicle by pumping hydraulic fluid into the accumulators and thereby compressing the contained gas medium. Alternately, the system is operable in a driving mode to supply supplementary power to the drive shaft of the vehicle using the pressurised hydraulic fluid from the accumulators. In the braking mode, it will be appreciated that the hydraulic pump/motor unit operates as a pump powered by the vehicle drive shaft, whereas in the driving mode, the unit operates as a motor powered by pressurised hydraulic fluid from the accumulators. The system is also operable in a third neutral or “free wheeling” mode, whereby the drive shaft of the vehicle is substantially unaffected by the pump/motor unit, aside from any residual frictional drag which the present invention aims to minimise.

[0084] In the neutral or free wheeling mode, the cylinder block may be marginally displaced from the valve plate by selective actuation of the lift-off cylinder, to minimise a key source of frictional power loss within the pump/motor unit. Simultaneously or alternatively, the locking collet may be selectively disengaged. As previously described, this avoids the need for the cylinder block to rotate in unison with the drive shaft when not required. The effect of this disengagement is to substantially eliminate a primary source of hydrodynamic drag which arises by virtue of the fact that the rotational group normally spins in an oil bath contained within the housing.

[0085] The three primary operational modes are controlled by the angle of inclination of the swash plate 15, in the manner previously described. This angle, together with the related operation of the hold-down cylinder, lift-off cylinder and locking collet mechanism, is regulated by a computer controlled electronic RDS management system 90. The RDS management system is programmably responsive to a predetermined series of system parameters such as accelerator, brake and clutch pedal positions, engine speed, gear selection, engine manifold pressure, swash plate position, drive line torque, accumulator pressure and hydraulic pump/motor pressure. The system may also be pre-programmed with topographical mapping and terrain data, thereby enabling it effectively to anticipate inclines and declines as well as stopping and acceleration points on known routes, and optimise the performance of the RDS on that basis. Again, the general operating principles in this respect are described in more detail in PCT/AU99/00740.

[0086] The RDS unit as illustrated is positioned in the central drive line of the vehicle, immediately downstream of the engine 92 and gearbox 94. This is advantageous, because in that position, the system can be readily retrofitted to existing vehicles by replacement of a standard section of the original drive line. It should be understood, however, that the unit may alternatively be positioned between the engine and gearbox, for example in direct connection with the crank shaft. As another alternative, it may be incorporated into a gearbox or engine casing. It may also be positioned downstream of one or both of the differentials 98, and may even be incorporated into an axle. In that case, it will be apparent that several RDS units may be incorporated into multiple drive line sections, axles or stub axles.

[0087] While the particular implementation of the invention shown in the drawings is described predominantly in the context of heavy road going vehicles, it will also be appreciated that regenerative drive systems of this type can be readily adapted to practically any environment in which it is advantageous to store excess mechanical energy at some time, for use at a later time. Typical examples of other potential applications include elevators and lifts, escalators and travelators, conveyors, cranes and other lifting devices, as well as other forms of transportation such as rail, shipping and aeronautical applications.

[0088] The present invention provides an efficient and effective mechanism for reducing frictional or hydrodynamic drag under off-load conditions in axial piston hydraulic pumps and motors, of almost any configuration and in virtually any application. Moreover, this can be achieved without compromising sealing performance under heavy load or high pressure conditions. The benefits of this are particularly substantial in the context of regenerative drive systems, wherein the pump/motor unit may be required to operate in a neutral or freewheeling mode for prolonged periods, and any residual frictional drag during such periods can impact materially on the overall efficiency of the system. In these respects, the invention represents both a practical and a commercially significant improvement over the prior art.

[0089] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 

1. A positive displacement hydraulic pump/motor assembly including: a rotary cylinder block having a central axis and incorporating a generally circular array of cylinders disposed in parallel relationship around the axis; a corresponding plurality of axial pistons reciprocably disposed within the respective cylinders; drive means to effect rotation of the cylinder block about the central axis; a drive plate disposed at one end of the cylinder block to effect sequentially staggered reciprocation of the pistons in response to rotation of the cylinder block; a stationary valve plate disposed at an opposite end of the cylinder block, the valve plate having a valve face adapted for sliding rotational engagement with a complementary mating face formed on the cylinder block; the valve plate further including at least one inlet port adapted for fluid communication with a source of hydraulic fluid and at least one outlet port adapted for fluid communication with an hydraulic load; the ports being disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate; the pump/motor assembly further including selectively releasable coupling means adapted in an engaged mode to connect the drive means to the cylinder block and in a disengaged mode to decouple the drive means from the cylinder block.
 2. A pump/motor assembly according to claim 1, wherein the drive means include a drive shaft, disposed in coaxial relationship with the cylinder block.
 3. A pump/motor assembly according to claim 1, wherein the drive shaft extends through a complementary bore formed in the cylinder block.
 4. A pump/motor assembly according to claim 1, wherein the drive plate takes the form of a stationary swash plate, which is inclined or inclinable with respect to the central rotational axis of the cylinder block.
 5. A pump/motor assembly according to claim 4, wherein the ends of the pistons remote from the valve plate include followers adapted to slide over the swash plate as the cylinder block rotates.
 6. A pump/motor assembly according to claim 5, wherein a hold-down plate is disposed to locate the floating ends of the pistons and retain the followers in sliding contact with the swash plate.
 7. A pump/motor assembly according to claim 5, further including springs to facilitate retention of the followers in contact with the swash plate.
 8. A pump/motor assembly according to claim 4, wherein the angle of inclination of the swash plate is selectively adjustable, to provide selectively variable flow rate characteristics.
 9. A pump/motor assembly according to claim 8, wherein the swash plate is adapted to be selectively inclined in a positive or a negative sense, thereby enabling the assembly alternately to operate as a motor or a pump.
 10. A pump/motor assembly according to claim 9, wherein the swash plate can also be oriented in an intermediate or neutral position effectively normal to the central axis, such that rotation of the cylinder block causes no axial movement of the pistons, hence induces no net flow into or out of the cylinders through the ports, and therefore induces no substantial load or drive on the shaft.
 11. A pump/motor assembly according to claim 1, wherein the pump/motor unit is a bent axis type hydraulic pump, further including piston connecting rods pivotably attached to a thrust plate adapted to rotate with the cylinder block.
 12. A pump/motor assembly according to claim 1, further including lift-off means being selectively operable so as axially to displace the cylinder block marginally away from the valve plate, thereby to minimise rotational resistance under predetermined operational conditions.
 13. A pump/motor assembly according to claim 12, wherein the lift-off means include a selectively operable hydraulic lift-off cylinder effectively interposed between the cylinder block and the valve plate, such that upon actuation, the cylinder block is axially displaced marginally away from the valve plate.
 14. A pump/motor assembly according to claim 13, wherein one end of the lift-off cylinder is in direct contact with a thrust bearing, to facilitate continued rotation of the cylinder block with minimal frictional resistance with the lift-off cylinder activated.
 15. A pump/motor assembly according to claim 13, wherein the lift-off cylinder is adapted to be activated with the swash plate in the neutral position, when there is minimal hydraulic pressure within the pump/motor assembly, thereby enabling selective minimisation of frictional drag between the cylinder block and the valve plate in situations where sealing of the valve ports in order to prevent leakage flow is non-critical to performance, because the pump/motor unit is doing no work.
 16. A pump/motor assembly according to claim 1, further including selectively variable bias means disposed to apply a variable bias force urging the respective mating faces on the cylinder block and the valve plate into sealing engagement.
 17. A pump/motor assembly according to claim 16, wherein said bias means include an hydraulic hold-down piston disposed within a complementary hold-down cylinder.
 18. A pump/motor assembly according to claim 17, further including a control actuator disposed to regulate pressure in the hold-down cylinder.
 19. A pump/motor assembly according to claim 17, wherein the hold-down cylinder is supplied by pilot pressure from the pump/motor unit.
 20. A pump/motor assembly according to claim 17, wherein the hold-down cylinder is supplied from a supplementary external source of hydraulic fluid pressure.
 21. A pump/motor assembly according to claim 17, wherein the hold-down piston takes the form of an annular sleeve disposed coaxially around the drive shaft, at an end of the cylinder block remote from the valve plate.
 22. A pump/motor assembly according to claim 18, wherein the control actuator is adapted to regulate the hold-down cylinder according to predetermined pressure and flow characteristics of the pump/motor assembly.
 23. A pump/motor assembly according to claim 22, wherein the hold-down cylinder is regulated such that under conditions of minimal motor or pump load, the pressure to the hold-down cylinder is released or substantially reduced so as to minimise frictional drag between the rotary cylinder block and the stationary valve plate, and such that under conditions of relatively high motor or pump load, the pressure to the hold-down cylinder is substantially increased, so as to minimise leakage between the cylinder block and the valve plate.
 24. A pump/motor assembly according to claim 23, wherein the hold-down cylinder is controlled in a linear, step-wise or other manner in response to pressure changes at selected points in the system.
 25. A pump/motor assembly according to claim 24, wherein the hold-down cylinder is further controlled wholly or in part according to other system parameters including one or more of: leakage flow; frictional drag; temperature changes; or combinations of such parameters.
 26. A pump/motor assembly according to claim 16, wherein the bias means include an actuating element selected from the group comprising:- an electromagnetic solenoid; a mechanical screw; an electrical actuator; a magnetic actuator; a mechanical linkage; or an hydraulic, pneumatic, electrical or mechanical hybrid actuating arrangement.
 27. A pump/motor assembly according to claim 16, further including a retention spring disposed to provide a minimum threshold level of substantially constant initial bias force sufficient to hold the cylinder block against the valve plate at relatively low pump/motor pressure levels, independently of the selectively variable bias means.
 28. A pump/motor assembly according to claim 27, wherein the retention spring takes the form of a preloaded annular bevel or frusto-conical washer disposed within an annular recess formed between the hold-down piston and the cylinder block.
 29. A pump/motor assembly according to claim 28, when dependent upon claim 17, wherein the threshold force provided by the retention spring is sufficient to allow the pump to supply a predetermined level of pilot pressure to the hydraulic hold-down cylinder with minimal leakage, at an initial non-zero swash plate angle.
 30. A pump/motor assembly according to claim 29, wherein the initial swash plate angle for the predetermined level of pilot pressure is between 0.1 degrees and around 5.0 degrees.
 31. A pump/motor assembly according to claim 30, wherein the initial swash plate angle for the predetermined level of pilot pressure is between 0.3 degrees and around 1.0 degree.
 32. A pump/motor assembly according to claim 31, wherein the initial swash plate angle for the predetermined level of pilot pressure is around 0.5 degrees off the neutral or zero position.
 33. A pump/motor assembly according to claim 1, wherein the coupling means include a releasable locking mechanism, adapted to enable selective disengagement of the cylinder block from the drive shaft.
 34. A pump/motor assembly according to claim 33, wherein the releasable locking mechanism includes a tapered locking collet disposed coaxially around the drive shaft.
 35. A pump/motor assembly according to claim 34, wherein the locking collet is splined internally for engagement with a correspondingly splined section of the shaft, so as to transmit rotary drive, while accommodating a limited degree of relative axial displacement between the collet and the shaft.
 36. A pump/motor assembly according to claim 33 when dependent upon claim 17 and claim 13, wherein the hydraulic hold-down cylinder, in addition to its function of applying a variable hold-down force, is configured positively to disengage the locking mechanism when the hydraulic lift-off cylinder is simultaneously de-pressurised.
 37. A pump/motor assembly according to claim 34, wherein an inner surface of the cylinder block is tapered to match a corresponding outer surface profile of the locking collet.
 38. A pump/motor assembly according to claim 37, wherein the outer surface of the locking collet is also splined, for engagement with complementary splines formed on the inner surface of the cylinder block such that upon axial engagement, the two components become mechanically interlocked for conjoined rotation.
 39. A pump/motor assembly according to claim 38, wherein the locking collet positively transmits rotary drive from the shaft to the cylinder block while allowing a limited degree of axial displacement of the cylinder block along the shaft, said limited degree of axial displacement, with the splines engaged, permitting the cylinder block to be alternately held positively against the valve plate by the hold-down cylinder and displaced marginally away from the valve plate by the lift-off cylinder, whereas upon full axial disengagement, the shaft is able to spin independently of the cylinder block.
 40. An energy management system operable in a driving mode, a braking mode and a neutral mode, said energy management system including: energy accumulation means operable selectively to store and release energy through controlled receipt and release of hydraulic fluid; a positive displacement pump/motor assembly, as defined in any one of the preceding claims, in fluid communication with the energy accumulation means; an hydraulic reservoir in fluid communication with the pump/motor assembly; and a drive shaft; the system being arranged such that in the braking mode the pump/motor assembly retards the drive shaft by pumping hydraulic fluid into the accumulation means, in the driving mode the pump/motor assembly supplies supplementary power to the drive shaft using pressurised hydraulic fluid from the accumulation means, and in the neutral mode the pump/motor assembly is effectively inoperative and exerts no substantial driving or retarding influence on the drive shaft.
 41. An energy management system according to claim 40, wherein the drive shaft forms part of the drive train of a vehicle.
 42. An energy management system according to claim 40, wherein the drive shaft extends through a complementary bore formed in the cylinder block of the pump/motor assembly, such that the drive shaft and the cylinder block are coaxial, with the pistons of the pump/motor assembly being uniformly disposed in parallel relationship around the drive shaft.
 43. An energy management system according to claim 40, wherein the coupling means include a connection adapted to transmit drive directly between the vehicle drive shaft and the cylinder block.
 44. An energy management system according to claim 43, wherein said connection includes spline formations formed respectively in the drive shaft and the cylinder block.
 45. An energy management system according to claim 43, wherein said connection includes a transmission interposed to transmit rotary drive between the vehicle drive train and the pump/motor unit.
 46. An energy management system according to claim 45, wherein said transmission includes mechanical, hydraulic, pneumatic or electromagnetic transmission elements.
 47. An energy management system according to claim 45, wherein said transmission is adapted to the permanently engaged.
 48. An energy management system according to claim 45, wherein said transmission is selectively decouplable.
 49. An energy management system according to claim 40, wherein the pump/motor assembly includes at least three external ports to permit ingress and egress of hydraulic fluid, with a first port communicating with an inlet of the hydraulic reservoir, a second port communicating with an outlet of the hydraulic reservoir, and a third port communicating with the accumulation means.
 50. An energy management system according to claim 40, wherein a heat exchanger is disposed between the first port and the hydraulic fluid reservoir.
 51. An energy management system according to claim 40, wherein a plurality of said pump/motor assemblies is arranged axially along the drive shaft, and connected hydraulically to operate in series, parallel, or a combination of both.
 52. An energy management system according to claim 40, further including a flow control circuit through which hydraulic fluid may be selectively directed, the control circuit being adapted to provide a controllable resistance enabling the pump/motor unit selectively to exert a retarding force on the drive shaft when required, irrespective of the charge state of the accumulation means.
 53. An energy management system according to claim 40, wherein the accumulation means include a gas/liquid accumulator comprising a double-ended cylinder and a piston adapted to float sealingly within the cylinder.
 54. An energy management system according to claim 53, wherein one side of the cylinder contains a compressible inert gas, while the other side of the cylinder is connected hydraulically to the pump/motor assembly, whereby the accumulator is adapted to store energy by pumping hydraulic fluid into one side of the cylinder, so as to compress the gas on the other side by displacement of the floating piston, and subsequently to release that energy by expulsion of hydraulic fluid as the compressed gas expands.
 55. An energy management system according to claim 40, wherein the accumulation means include a bladder or diaphragm type accumulator.
 56. An energy management system according to claim 40, wherein the accumulation means include a plurality of accumulators connected in series.
 57. An energy management system according to claim 40, wherein the accumulation means include a plurality of accumulators connected in parallel. 